1. Technical Field
The present invention is directed to an organic light-emitting device and a thin film transistor which are monolithically integrated. The present invention is also directed to a process for making the device.
2. Art Background
Light emitting diodes (LEDs) with an organic layer as the active layer have been developed for use in flat panel displays. LEDs with organic thin films are attractive because such devices need not be produced on a crystalline substrate, the cost of producing such devices is low, the devices operate at low voltage, and the organic thin films enable devices that emit light in a variety of colors to be produced. Consequently LEDs are attractive for use in making flat panel displays that are unbreakable, lightweight, flexible, and inexpensive.
Thin-film transistors (TFTs) have been used as the switching elements in liquid crystal displays. As noted in Wu, C., et al., "Integration of Organic LED's and Amorphous Si TFT's onto Flexible and Lightweight Metal Foil Substrates," IEEE Electron Device Letters, Vol. 18:12, pp. 609-612 (1997), there is a desire to integrate TFTs and LEDs for making active-matrix emissive flat panel displays. The device 10 described in the Wu et al. article is illustrated in FIG. 1. Both the LED 11 and the TFT 12 are formed on a stainless steel foil substrate 13. The TFT is formed by depositing an insulating layer 14 over the foil substrate 13. A chromium gate 15 is formed on the substrate, over which is formed another insulating layer 16.
A layer of undoped amorphous hydrogenated silicon 17 is formed on the insulating layer, over which is formed a layer of n.sup.+ amorphous hydrogenated silicon 18. Chromium source/drain contacts 19 are formed on the doped silicon layer 18. After the TFT is fabricated, the LED 11 is formed by depositing a platinum anode 21 on the source/drain contact 19. A layer of luminescent polymer 22 is formed over the source/drain contact 19 and the platinum anode 21. The cathode (double-layer Mg:Ag(10:1)/ITO) 23 is formed on the surface of the luminescent polymer and placed so that the luminescent polymer is sandwiched between the anode 21 and the cathode 23.
Wu et al. note the advantages for producing an integrated TFT and LED on a plastic substrate. However, Wu et al. farther note that such a device would be difficult to fabricate due to the mechanical and chemical instabilities of such substrates. The integrated device described by Wu et al. essentially requires that the TFT be formed first on the substrate followed by formation of the LED.
The desirability of a TFT driving an LED with an organic layer is also noted in Hatalis, M., et al. "Polysilicon TFT active matrix organic ETL displays," SPIE Vol. 3057, pp. 277-286 (1997). Hatalis et al. describes integrating a polysilicon TFT with an organic LED on a glass substrate. The TFT structure described by Hatalis et al. has a doped polycrystalline silicon source and drain regions formed on the glass substrate. The gate dielectric material is silicon dioxide formed by heating the substrate with the polycrystalline silicon formed thereon to 1000.degree. C. in an oxygen containing atmosphere. The gate is also doped polycrystalline silicon. A layer of silicon dioxide is formed over the TFT device, with contact windows etched through the passivation layer to the source and drain regions and filled with aluminum. The data lines formed on top of the passivation layers is patterned aluminum. Another layer of passivation oxide is formed over the aluminum with vias etched therein for the pixel contacts. A patterned indium tin oxide (ITO) layer is formed over the passivation layer. The active organic material is formed over the ITO, over which a top electrode is formed.
The device described in Hatalis et al. is also difficult to fabricate. Specifically, the fabrication requires high temperature processing and numerous lithographic and etching steps to form the vias.
Accordingly, a monolithically integrated LED and TFT in which the LED and TFT are formed using a simplified process is desired.